The induction and regulation of retroviruses in reproductive tissues during pregnancy and their relationship with growth regulating factors during prenatal ontogeny are being examined. Interferons (IFN), mitogenic stimulating factors (MSF) and colony stimulating factors (CSF) have been identified in murine gestational tissues and in vitro model systems developed to characterize the bioregulatory activities of these factors, singularly and in combination. IFN are detected in placentas of fetuses as early as the 13th day of pregnancy, then their levels progressively increase to term. Biophysical characterization of antiviral activity by gel filtration indicates the presence of at least two components, one in the molecular weight region of virus induced (Type 1) IFN. The relative level of placental interferon is under maternal genetic control and its production appears to involve an adherent cell population.